1. Field of the Invention
The present invention relates to wire-wound coils, and more specifically, to a wire-wound coil capable of reducing magnetic flux leakage to improve the efficiency of obtaining inductance.
2. Description of the Related Art
Known wire-wound coils of this type include common-mode choke coils described in, for example, Japanese Unexamined Patent Application Publication No. 2003-168611, Japanese Unexamined Patent Application Publication No. 10-163029, and Japanese Unexamined Patent Application Publication No. 2005-056934. As shown in, for example, FIG. 10A, a high-frequency common-mode choke coil described in Japanese Unexamined Patent Application Publication No. 2003-168611 includes a core 1 having a winding core portion 1A and flange portions 1B, a wire 2 wound around the winding core portion 1A of the core 1, electrodes 3 provided on lower surfaces of the flange portions 1B of the core 1, and a ferrite plate 5 adhered by an adhesive 4 to upper surfaces of the both end flange portions 1B so as to extend over the winding core portion 1A. The ends of the wire 2 are electrically connected to the electrodes 3.
In a common-mode choke coil described in Japanese Unexamined Patent Application Publication No. 10-163029, as shown in FIG. 10B, flange portions 6A at both ends of a soft-magnetic drum core 6 are provided with projecting portions 7 for electrodes, and the projecting portions 7 for electrodes are provided with electrodes (not shown). A soft-magnetic return magnetic path member 8 having a substantially U-shaped configuration and the drum core 6 are engaged with each other to define a closed magnetic path. In FIG. 10B, reference numeral 9 denotes pullout lead portions at both ends of a coil winding.
Although not shown in the figures, a common-mode filter described in Japanese Unexamined Patent Application Publication No. 2005-056934 has a structure that is similar to that of the common-mode choke coil of Japanese Unexamined Patent Application Publication No. 2003-168611. However, this common-mode filter is different from the high-frequency common-mode choke coil of Japanese Unexamined Patent Application Publication No. 2003-168611 in that flange portions of a core and a ferrite plate are joined to each other by an adhesive through recessed and projecting portions.
In the high-frequency common-mode choke coil of Japanese Unexamined Patent Application Publication No. 2003-168611, since the flange portions 1B of the core 1 and the ferrite plate 5 are joined and fixed by the adhesive 4, a space having no magnetic material, which corresponds to an adhesive layer, is disposed between the ferrite plate 5 and the flange portions 1B. Magnetic flux leaks from this space, and the efficiency of obtaining inductance is reduced. For a similar reason, the common-mode filter of Japanese Unexamined Patent Application Publication No. 2005-056934 also causes magnetic flux leakage.
Also in the common-mode choke coil of Japanese Unexamined Patent Application Publication No. 10-163029, similar to Japanese Unexamined Patent Application Publication No. 2003-168611 and Japanese Unexamined Patent Application Publication No. 2005-056934, magnetic flux leakage occurs because the flange portions 6A of the drum core 6 and the U-shaped return magnetic path member 8 are adhered and joined by an adhesive. Japanese Unexamined Patent Application Publication No. 10-163029 describes that gaps corresponding to the joint portions between the flange portions 6A and the return magnetic path member 8 are minimized. If the gaps are minimized, however, the joining strength between the drum core 6 and the return magnetic path member 8 is reduced, resulting in reduced reliability. A positioning projecting portion (not shown) is further provided between the drum core 6 and a wire-winding portion to facilitate engagement between the drum core 6 and the return magnetic path member 8 to easily join the drum core 6 to the return magnetic path member 8. The return magnetic path member 8 and the positioning projecting portion limit the space for winding the wire. It is therefore difficult to use a thick wire and a desired number of winding turns may not be obtained.